Hydrostatic tool holders, such as that disclosed in U.S. Pat. No. 5,516,243 have been used to clamp or hold workpieces or work tools within a sleeve which is urged into engagement with the workpiece by a hydraulic or hydrostatic pressure acting on the sleeve. Such hydrostatic holding devices are useful to accurately, repeatably and securely hold a workpiece or work tool and therefore have had great commercial success. The hydraulic pressure which urges the sleeve into engagement with a workpiece or work tool is supplied by an external hydraulic source or by rotation of a screw disposed in a passage filled with a hydraulic fluid to compress the hydraulic fluid and thereby increase its pressure.
One prior application for a hydrostatic holding device is to hold a workpiece carried by a pallet of a conveyor to successive work stations whereat the workpiece is subjected to successive machining operations. To firmly hold the workpiece, an external hydraulic source is connected to the hydrostatic holding device to provide the pressurized hydraulic fluid which urges a sleeve into engagement with the workpiece. After a sufficient pressure is achieved within the hydrostatic device, the hydraulic source is disconnected from the holding device. The workpiece firmly clamped in the hydrostatic device is then moved on a pallet of a conveyor to the various work stations. After the workpiece has been formed, a hydraulic reservoir must be connected to the hydrostatic holding device to bleed the pressurized fluid from the device and thereby relax the sleeve to permit the workpiece to be removed from the holding device. This process is repeated for each workpiece and while functional, is inefficient, time consuming and labor intensive.